I'm not sure if this is the easiest way, but probably via CIImageRep's (NSImageRep's) drawInRect: method. There are quite a few different ways of capturing that output into various objects, the easiest of which is probably NSImage with something like this (untested):

also how is the information stored in outputPixels? can I create a NSBitmapImageRep object from outputPixels? to have a method to get the color of the pixel (x,y), a NSColor or something I can work on, or can I get somehow that information from outputPixels directly?

Whoops, I forgot to replace BYTES_PER_PIXEL in my paste with its actual value, which is 4. outputPixels ends up as a linear array of bytes representing pixel components (four per pixel, premultiplied), laid out as R, G, B, A, R, G, B, A, ..., so you can just look up color values directly from the array.